Wood-burning fireplaces have been used for centuries in homes throughout the world. They have provided a means of heating and cooking. More recently, they have taken a more aesthetic role as more efficient and convenient means of heating and cooking have evolved. Today, fireplaces are considered a desirable feature in any home and are often the gathering point or focal point for special occasions. It is often said that the warmth and security they provide fill an instinctual need left over from man's earliest ancestors. In any case, people are simply drawn to an open fire. The number of existing fireplaces in the United States is estimated in the tens of millions. The number of new fireplaces installed in the United States is estimated in the hundreds of thousands each year.
Wood combustion does have its drawbacks. The incomplete combustion of wood can result in various forms of air pollution. For example, volatile organic materials may be released during the combustion process (referred to a pyrolysis) and if not substantially oxidized (burned) before entering the chimney, wood smoke is produced. Notable among the pollutants comprising wood smoke is fine particulate matter of a size that is easily respirated into the deepest parts of the lungs. The potential health impacts from this are well documented. For this reason, residential wood burning has come under close scrutiny. For example, Oregon, Colorado, Washington State and the U.S. Environmental Protection Agency have been regulating particulate emissions from several categories of wood-burning devices since the mid-1980's.
However, these regulations, for the most part, and the EPA regulations specifically, have excluded traditional wood-burning fireplaces. This was done for several reasons. It was recognized that the vast majority of wood-burning fireplaces are used on a very infrequent basis and primarily for aesthetic enjoyment. The total amount of wood consumed in fireplaces was low when compared to other residential wood-burning devices such as wood stoves that are used for heating. This meant that the total contribution, relative to other known sources, was quite low. Also, at the time the EPA regulations were being formulated, it was recognized that there was no viable particulate emission control technology that was applicable to fireplaces.
As an increasing number of air quality regulators have begun to face more stringent National Ambient Air Quality Standards for fine particulates in their jurisdictions, they have been forced to start looking at other lower level sources and specifically at sources that are currently uncontrolled. Traditional wood-burning fireplaces fall into this category. It is generally recognized that traditional wood-burning fireplaces must begin to show a significant improvement in particulate emission performance or face the possibility that they will no longer be able to be built or installed in wide-ranging areas throughout the country.
The wood-burning emission control development efforts for the past 25 years have been focused almost exclusively on wood-burning stoves and it is generally understood by those skilled in the art that those control technologies can not be directly applied to fireplaces due to the fundamental differences in the size, design and use patterns between the two. Woodstoves have relatively small fireboxes, are generally batch-loaded with nearly full fuel loads and have tight-fitting load doors.
Fireplaces, on the other hand, have much larger fireboxes, more properly called a fireplace cavity and may be fueled with small fuel loads relative to the total fireplace cavity volume. Some fireplaces may include fire screens (to contain sparks and embers) in front of large front fireplace openings. Large amounts of air may enter the fireplace cavity through the open face of the fireplace. Some of the air entering near the bottom of the fireplace flows to the burning fuel load providing the oxygen needed to sustain combustion of the fuel. Some of the air entering at the upper portions of the fireplace opening simply bypasses the actual combustion occurring in and around the fuel load and flows over the fire and directly up the chimney, providing the benefit of sweeping up and carrying away any stray smoke before it can spill into the room. However, this air also can have the detrimental impact of diluting and quenching the natural secondary combustion of the gaseous and volatile organic materials emanating from the burning fuel load.
The aesthetically pleasing yellow flames that are seen propagating above an actively burning fuel load are the natural secondary combustion of the gases and volatile materials. When excess air mixes with those flames, the temperature drops and combustion is halted before all of the combustible materials have been completely burned. The unburned materials form the smoke (and unwanted pollutant materials) that you see exiting the chimney. Even those fireplaces with typical bi-fold glass doors on the front do little to control the large volumes of air being drawn into the fireplace and therefore have only minimal impact in improving the combustion environment. This continuous high excess air condition represents the primary difference between wood-burning stoves and wood-burning fireplaces and is the reason that particulate emission control technology that has been shown to be very effective in wood stoves will not translate directly to fireplaces.
Since air flow is not readily controlled in a fireplace, other ways of dividing and guiding air to the needed locations while diverting some of the unwanted excess air away from the fire must be employed. If this can be accomplished, in combination with providing a favorable environment in terms of temperature and mixing, secondary combustion of the pollutant emissions can be initiated and sustained over a significant portion of the fireplace burn cycle. Thus, there exists a need for a secondary combustion system for wood burning fireplaces that is configured to assist in controlling air flow to maximize secondary combustion.